1. Field of the Invention
The invention pertains to a device for supplying power to a filament, especially that of an X-ray tube such as is used in X-ray diagnosis equipment. The invention is especially applicable to cases where a wide range of current values has to be supplied successively to filaments with very different resistance values.
2. Discussion of Background
An X-ray tube for medical diagnosis is generally set up like a diode, i.e. with two electrodes, one of which, called a cathode, emits electrons while the other is called an anode and receives these electrons on a small area which is the source of X-radiation.
The cathode comprises a heated filament which constitutes the source of electrons. When the high voltage supplied by a generator is applied to the terminals of both electrodes, so that the cathode is at negative potential, a so-called anode current is established in the circuit, through the generator, and crosses the space between the cathode and the anode in the form of a beam of electrons, the intensity of which depends on the temperature of the filament, this temperature depending on the power dissipated in the filament i.e. on the current, called the heating current, which flows in the filament.
The quantity of X-rays emitted by the anode depends chiefly on the intensity of the anode current and, hence, on the intensity of the filament-heating current.
Thus, the filament-heating current is one of the major parameters which must be determined for each radiographic or radioscopic exposure during an X-ray examination of a patient.
The parameters of the exposure are determined according to the nature of the examination. These parameters are generally pre-determined by an operator who sets their values on a control panel which controls the functioning of the various elements of an X-ray diagnosis installation such as, for example, the high-voltage generator and the generator of filament-heating current. Usually, in certain installations, the values of these parameters are pre-determined by means of a microprocessor-based device which may or may not be built into the control panel and which calculates and programs the optimum values of these parameters according to, for example, the type of examination desired by the practitioner and according to the specific characteristics of the installation.
In all cases, this operation particularly involves programming different values such as, for example, the length of the exposure time, the energy of the X-radiation by choosing the value of the high voltage applied between the anode and the cathode, and the intensity of the anode current by choosing, in particular, a value of the filament-heating current intensity.
It must be noted that the intensity of the heating current can be substantially altered from one exposure to the following one, for example, from 1.5 amperes to 5.5 amperes.
Furthermore, X-ray diagnosis installations usually include several X-ray tubes with different characteristics, which are successively put into operation, sometimes during the same examination. These X-ray tubes may comprise filaments, the ohmic resistance value of which may vary considerably from one tube to another, from 0.6 ohms to 4.5 ohms for example. In such cases, it is especially worthwhile to have a heating-current generator which can be used to quickly, i.e. automatically, obtain a heating current value within the range of values referred to earlier, regardless of the resistance value of the filament supplied with current.
Consequently, the generator which produces the heating current must supply this current in a very extensive range of power. Furthermore, within this range of power, it must ensure quality which is adequate for the regulation of the heating current, and must make it possible, quickly and automatically, to obtain the desired intensity value as defined, for example, by a set value. This set value may vary between successive exposures.
Heating-current generators according to the prior art cannot be used to obtain these conditions satisfactorily, because either they require manual adjustments depending on the intensity of the heating current and the resistance value of the filament or they provide for wide-ranging power to the detriment of the quality of regulation. Furthermore requirements in terms of power range, automation system and quality of regulation may result in the designing of complex generators, i.e. generators that are fragile, hardly reliable or bulky and expensive.
It must also be noted that the regulation of the filament-heating current is further complicated by the fact that the cathode and the filament of the X-ray tube are connected to the high voltage negative potential. Hence, the problems of electrical insulation generally lead to the application of heating energy to the filament by means of an isolating transformer, the primary winding of which represents the charge of the filament. As a result the heating current is produced according to an alternating current, for which the measurement of the root-mean-square value can also present problems.